Plasticized polyvinyl butyral is widely used as an interlayer or insert in the laminated safety glass used particularly in automobilies or building constructions. The laminated safety glass used for these purposes should exhibit certain properties such as good edge stability, heat resistance, and resilience, particularly at low ambient temperatures. The plasticized polyvinyl butyral interlayer determines the degree to which these desired properties are exhibited by the laminated safety glass.
The edge stability, or resistance to delamination of the laminated safety glass, is directly linked to the compatibility of the plasticizer with the polyvinyl butyral resin (PVB). Incompatibility of the plasticizer and the PVB resin results in the exudation of the plasticizer from the plasticized polyvinyl butyral sheet and subsequent delamination of the laminated glass.
The use of mixed alkyl alkylaryl adipates as PVB plasticizers is known in the prior art, European Pat. No. 11577. These adipates are generally represented by the following structural formula: ##STR1## wherein n is greater than or equal to 1 and the sum of p+r is greater than or equal to 1.
Typical mixed adipates of this formula are benzyl octyl adipate, benzyl hexyl adipate, benzyl butyl adipate and benzyl decyl adipate.
These adipates are suitable plasticizers for a PVB resin having a molecular weight of 30,000 to 600,000, comprising 0 to 10% by weight of residual ester groups, calculated as polyvinyl acetate and 12 to 25% by weight of hydroxyl groups, calculated as polyvinyl alcohol. Mixed adipates of this type demonstrate a good compatibility with the PVB resin. Thus, laminated glasses containing these plasticized polyvinyl interlayers exhibit good mechanical properties and have excellent edge stability and good heat resistance. For certain uses, however, these mixed adipate plasticized polyvinyl butyral interlayers fail to provide the laminated safety glass with adequate cold resistance and particularly cold resilience.
The use of dialkyl adipates, such as dibutyl adipate, di-n-pentyl adipate, di-n-heptyl adipate and di-n-octyl adipate as PVB plasticizers is also known. These plasticizers exhibit only a partial compatibility with the PVB resin that decreases as the molecular weight of the adipate and/or the hydroxyl group content of the PVB resin increases. Thus, plasticizers of this type are effective only if they consist of small molecular weight dialkyl adipates or if the PVB resin has a low hydroxyl group content. Under these circumstances, the heat resistance and/or the edge stability of the laminated glass containing these plasticizers is unsatisfactory.